The present invention relates, in general, to diaphragm pistons that operate in the cavity of a body in the manner of a piston and cylinder and, in particular, to such diaphragm piston arrangements in which the diaphragm exhibits an inherent "spring effect", which can be beneficial particularly when the diaphragm piston is employed to operate pneumatic valving and the like.
In railroad brake control applications, where it is common practice to employ high pneumatic pressures on the order of 100 psi., for example, a fabric-reinforced-type of diaphragm is necessary to withstand the high-pressure forces without diaphragm "balooning" and subsequent failure. These fabric-reinforced diaphragms tend to be stiffer than regular diaphragms and thus exhibit a substantially noticeable "spring effect". This so-called "spring effect" is an inherent force within the diaphragm itself when the diaphragm is forced to assume a configuration other than its molded-in or normal configuration. This force typically acts in a direction to restore the diaphragm to its normal molded-in configuration. In sensitive operating control valves, as in the well-known, industry standard, ABD type railroad brake control valve device, in which the service valve is comprised of a diaphragm-type piston that positions a slide valve to achieve the desired brake control and functions in response to variations in the brake pipe/auxiliary reservoir pressure relationship acting across the diaphragm piston, it is desirable to actuate the piston at very low pressure differentials in order to position the slide valve and achieve the resultant control function without delay. This is particularly desirable in actuating the piston from its release position, as shown in FIG. 2 of the drawings, to its application position, as shown in FIG. and relies upon the diaphragm "spring effect" to help achieve this purpose.
Because of the relatively long service life required of diaphragms used in the above-mentioned application, conical-type diaphragms, as disclosed in U. S. Pat. No. 3,173,342 and incorporated herein by reference, are typically employed. The significantly long service life attributed to these conical-type diaphragms is achieved by maintaining the fabric material uniformly embedded in the rubber that comprise the diaphragm proper. This is possible since the normally flat fabric material is not required to assume an unnatural or convoluted shape during the molding process, as in bellows-type diaphragms, for example, and therefore does not tend to shift toward the surface of the rubber. In realizing a long service life, however, due to the fabric material in conical-type diaphragms being unstressed during the vulcanizing process, these conical-type diaphragms also exhibit a relatively light "spring effect" for the same reasons.
Consequently, the efficiency of the control valve device employing such conical-type diaphragms is compromised with respect to achieving fast brake response. Moreover, the convolution in these conical-type diaphragms has been found to take an inside-out set over a period of time, which further reduces the diaphragm "spring effect" and contributes to the decline in brake response.
It will be understood, for example, that during a brake release, a relatively high pressure differential is created across piston 2, thereby causing convolution 17 of diaphragm 1 to become inverted during movement of piston 2 from application position to release position, as shown in FIG. 2. It is important to note at this point that the bend formed at clamping bead 10 between the outer diaphragm periphery and convolution 17 is essentially 90.degree.. Diaphragm 1 is provided with internal stress due to this bend, in addition to the stress due to the diaphragm convolution. Once movement of piston 2 to brake release position is complete and the pressures across piston 2 have become substantially equalized, the inherent "spring effect" of diaphragm due to the internal diaphragm stresses is intended to gradually force the diaphragm convolution 14 to automatically unfold or revert back to its normal upward disposition, as shown by the dotted lines of FIG. 2. Piston 2 is, therefore, in readiness for immediate actuation to application position in terms of the "spring effect" being in the desired direction to encourage movement of piston 2 toward application position. Also, the volumetric displacement between the pressure chambers on opposite sides of the diaphragm piston, due to transition of the diaphragm convolution, will have occurred prior to a subsequent reduction of brake pipe pressure when a brake application is desired.
In the event, however, the diaphragm convolution 17 does not revert back to its normal position following release of a brake application; and, since the piston normally remains in release position for a considerably long period of time between brake applications, the diaphragm convolution 17 tends to take a set in a downward disposition, as shown by the solid lines in FIG. 2. The result of this is that the initial upward-acting "spring effect" is lost, and the set resists further diaphragm movement. Consequently, a higher than normal pressure differential is required to actuate piston 2 when a brake application is subsequently initiated, thus increasing response time and adversely extending the time required to obtain braking. This condition is aggravated by the fact that the conventional clamping arrangement of the ABD type control valve service piston diaphragm, as shown in FIGS. 1 and 2, predisposes the diaphragm convolution in a downwardly-directed dispostion by reason of the clamping face of cover 3 urging the outer periphery of diaphragm 1 into engagement with the tapered surface of clamping bead 10.
The object of the present invention, therefore, is to provide a diaphragm clamping arrangement for a diaphragm piston that increases the internal diaphragm stress when the diaphragm convolution becomes inverted to better encourage the diaphragm convolution to revert to a predetermined disposition corresponding to the direction of the diaphragm "spring effect".
Briefly, this objective is achieved by providing a conical surface on one of the upper and lower members between which the outer diaphragm periphery is clamped. This conical surface lies adjacent a clamping bead formed on the other body member, so that when the upper and lower body members are tightened down, the diaphragm is forced to follow an angle corresponding to the slope of the conical surface against which it is clamped by the clamping bead. By sloping the conical surface in a direction to urge the diaphragm convolution in an upward direction, i.e., in a brake application direction, a sharper than normal bend is formed about the diaphragm clamping bead when the diaphragm convolution becomes inverted. This produces a greater internal stress on the diaphragm tending to cause the diaphragm convolution to revert to its normal disposition, even when conical-type diaphragms that typically exhibit a low "spring effect" are employed.